Acoustic reverberation arrangements



Feb. 2, 1960 w. K. KUHL ACOUSTIC REVERBERATION ARRANGEMENTS 2 Sheets-Sheet 1 Filed July 26, 1956 Album/{Mall Feb. 2, 1960 w. K. KUHL 2,923,369

ACOUSTIC REVERBERATION ARRANGEMENTS Filed July 26, 1956 2 Sheets-Sheet 2 United States Patent 2,923,369 ACOUSTIC REVERBERATION ARRANGEMENTS Walter K. Kuhl, Nuremberg-Eibach, Germany, assignor to Wilhelm Franz K.G., Lahr, Baden, Germany, a firm Application July 26, 1956, Serial No. 600,290 Claims priority, application Germany August 11, 1955 7 Claims. (Cl. 181-31) When picking up sound in broadcasting and television, for films and records, and in the case of electronic music, any reverberation which is lacking is frequently added or the existing one is enlarged (or increased). lfor this purpose, the electro-acoustically picked up sound is, after amplification, radiated into a reverberation chamberwith long reverberation time, is again picked up with this reverberation and added to the primary sound, that 1s to the direct sound, with the required level difference. The known reverberation chamber is open to the following objections:

(1) When it has the necessary volume of 60 to l 0 cubic meters, it is relatively expensive, especially if it is constructed with an elastically mounted inner shell, which is mostly necessary in the interests of high sound lIlSlllation.

(2) To ensure that the level in the diffuse sound field decreases only slightly at high frequency, a very expensive loudspeaker combination composed of many lndlvidual systems radiating in different directions and an electr c filter must be provided in addition to a large power amphfier.

(3) The reverberation time can be reduced only with a relatively high outlay and cannot be reduced in a short time, so that this possibility is not generally made use of. As a result, when short reverberation time is desired, the reverberation picked up in the sound room (with long reverberation time) is added to the direct sound with a great level difference, with the result that unsatisfactorrly bent reverberation curves are obtained.

(4) Most reverberation chambers have too little natural frequencies in the case of low frequencies (to about 150 cycles per second).

The second method is to produce the reverberation art1- ficially. Many methods have been suggested and even put into practice to a small extent for this purpose. They can, for example, be divided into the following groups: v

(l) The number of repetitions (reflections) of an im pulse-like primary sound in a given unit of time is constant. This group includes the one-dimensioned acoustic sound delaying methods by means of tubes (filled with air or liquids) or a rod (transferring structure-borne sound as in Patent No. 2,512,130), and the one-dimensioned propagation of bending Waves into helical metal springs (US. Patent No. 2,230,836), the disadvantages of which include, in addition to those hereinafter set forth, also the narrow frequency band width, the unsuitable frequency curve of the reverberation time and the low number of natural frequencies. The quasi one-dimensional propagation of sound in a relatively large room between a pair of parallel reflecting walls while the other two pairs of walls are absorbing can also be included among these arrangements. The simple sound delaying arrangements with the aid of magnetic recording also come in this group. A common feature of all the methods described is that the individual reflections are equidistant or substantially equidistant in time, with the result that, in the case come considerably closer of impulsive sound, the known flutter echo or a rattling reverberation but no really continuous reverberation is produced. I

"the reverberation arrangement given in US. Patent No. 2,437,445 discloses ondy a variation of the device according to US. Patent 2,230,836 in which the two coil springs together with the very light rotary suspension, which reflect only to a small extent, form a close mechanical system. Furthermore, the mechanical damping is constructed in another way in the form of rotary forks providing a damping coating for changing the reverberation time. The above described disadvantages that the series of reflections do not come periodically closer as time progresses, and the too small number of natural frequencies as well as the too narrow band-width are also inherent in this arrangement.

These methods only produce a reverberation which will also satisfy the exacting requirements in the case pulsive sound, if the intervals between the reflections are filled by additional arrangements which have become known in the meantime, namely by reflections which beas time progresses or by an actual reverberation process.

(2) The number of reflections in a given period of time increases proportionally to the time. As practical experiments have shown, the subjective acoustic impression is, in this case, equivalent to the case of the sound waves in a room (flat space) which is large in two dimensions but small in the third, for all wave lengths coming into question, so that the velocity of sound in the room is independent of frequency. If the last condition is fulfilled, the wall absorption by wall'friction is so great both in the case of air filling and also in the case of water filling, that a satisfactory long reverberating time is not obtained. A fiat that is a plate, which is excited by longitudinal waves can have sufliciently. small sound absorption. required that the natural vibrations are so dense that, for example already at cycles per second, a natural vibration falls to each musical quarter tone, that is to every 3 cycles, a steel or aluminium plate must have a surface area of about 30,000 square meters. Such an arrangement is consequently not practicable even in the case of moderate requirements. a

The linear increase in the reflections with the time can also be attained in a complicated manner and with expensive apparatus, if two delaying devices of the first group, for example devices operating according to the magnetic sound-recording method, with diflerent delaying times are series connected in cascade. As, however, the simpler and much cheaper arrangement according to the invention achieves the same or even better results, the complicated method is not used in practice.

(3) The number of reflections in the unit of time increases proportionally to the square of time. This group includes all processes with three-dimensional acoustic wave propagation in a space, which in the case of airborne sound is the known reverberation chamber, the objections to which have been set forth above. Operating with sound conducted through liquids and solids, is, on account of the many timesgreater sound velocities, possible only with practical dimensions if the sound is transposed from the audible sound range to the supersonic range, which has frequently been suggested and also carried out.

of im-' room or enclosure of solid material,

In order to obtain the above-mentioned meter would haveto be chosen in the case of steel and aluminium with a carrier frequency of 100 kilocycles per second. (Weight in the case of steel would be three tons.) However, with such a high frequency a reverberation time of 3 seconds, which is the least required as upper limit, is not attainable in very small specially treated pieces and much less in such a large block which moreover cannot be produced free from shrink holes and with smooth surface. This reverberation timewould be possibl at 30. kilocycles per second, but the steel blockwould hav a. weight of 30 tons. This method of transposing is therefore only practicable with sound conducted through liquid, that is, for example, in a Water-filled metal tank free from attenuation, both on account of the necessary number of natural frequencies and also of the reverberation time. The last process is open to the followingobjections: i V i (a). The. liquid evaporates the tank must be hermetically closedon all sides;

"(b). the liquid must be freed from air before being filled into. the tank sothat no gas bubbles can form which would. increase the magnitude of attenuation;

(c),if, when there is no air-tight closure, the' liquid again becomes saturated with airin the course of time, gas bubbles can again form when the temperature rises;

(d) it is necessary to transpose thesound twice which is to be provided with reverberation; Moreover, the carrier or the carrier and-the second side band must not beradiated into thetank; to prevent which, additional electrical equipment, possiblyv with quartz filter, is' neces- Nowthe present invention relates to an arrangement by, which the objections of the arrangements mentioned are avoided. It operates with a so id body very exten sivein two dimensions and very thin in the third, that is .a so-lidplate. The plate is not excited to density or longitudinal waves but to bending waves. Whereas the sound velocityof the density or longitudinal waves is only dependent upon the characteristics of the material,

density and elasticity, and not upon. the frequency,'the.

bending wave velocity is additionally dependent upon the thickness. of the plate and the frequency. 'In the case of high frequencies it approaches asymptotically the value of the density wave speed. In the frequency range which isof interest, the equation can be said to be B=\/1-8 LX Xf wherein c =1ongitudinal wave speed,-d=plate thickness, f=frequenCY., For very low frequencies the speed of the bending waves is in the order of 1000 times smaller than that of the longitudinal waves. Therefore, in order to obtain the corresponding natural oscillation of the plate in the case of excitation to bending vibration, as in the'case of -longitudinal excitation, the plate needs only 1000 times smaller dimensions... Only on account of the low speedofthe bending waves is it possible to produce a reverberation arrangement at all, the active elementof which, isa plate excited by bending vibrations without it be ng necessary to transpose the sound into a higher frequency range. The interval between adjacentnatural frequencies ofthe-plate is substantially constant and in.

order ofmagnitude amounts toa few cycles per second. One of the advantages of the arrangement according to the'invention is therefore the fact that. therequirement that several natural frequencies fall, to a half tone, can easily be realized right down to the lowest frequencies.

The thickness of the plate is-for instance in the order of. 1 nm.,,. h.e. l ng h. nd widthinthe-orderor 1; in. Length-- and width are preferably different, so. that many natural vibrations donfot repeatedly fall on'thej same frequencies.

If the plate is made from metal free. fromattenuation and the suspensioniis suitably chosenIthemagnitude and frequency response of the reverberation time can attain oreven exceed those of the largest. and most reverberant in the course of. time, or

be effected by various methods, namely by means of one or several loudspeakers over a coupling air cushion or directly by means of one or several transducers for structure-borne sound. Dynamic systems, as in the case of loudspeakers for air-borne sound, have proved most satis- -factory, primarily on account of the favourable frequencyresponse curve. The same applies in connection with the picking-up of sound provided with reverberations from the plate. This is also possible by means of microphones for air-borne or structure-borne sound. in the case of the latter, piezo-electric microphones are most suitable. It must be taken into consideration when picking up airborne sound that below the so-called limiting frequency, which in this case lies at the upper and of the audio frequency range, the plate does not radiate sound waves but only a stationary sound field is present in front of the plate, which field dies out in phase exponentially with the distance, on which account the microphone or microphones should be arranged close up.

One of the greatest advantages of the acoustic reverberation arrangement according to the invention is the possibility of varying the reverberation time using different methods by means of a handle. The attenuatlon of the plate can be effected by solid bodies, liquids or by absorbing materials for air-borne sound. Thus, for example, one or several thin rubber plates or sheets, which in und amped position do not touch thereverberation plate, but first come into contact therewith. at one point on the edge, whereafter this pointof contact gradually lengthens until the sheet bears againstthe entire length of the edge under slight tension. The reverberation time gradually changes overfrom that of the undarnped plate to relatively small values. The attenuation is stronger in the case of low frequencies than of high frequencies. Any other contacting of the plate increasing in steps or gradually with solid damping-materials represents a reduction in the reverberation time in the sense of the invention.

The plate may also be dipped-into a damping liquid commencing, forexample, at one corner and extending "mum attenuation is influenced by the viscosity of the case of a good concert hall or studio.

liquid. I 1

The} most advantageous form of attenuation, both as regards the extent and also the frequency-response, is that attained by approaching porous sound absorbing plates towards the reverberation plate without direct mechanical contact.

more. The best results areobtainable withathin layer of which is in theorder of the characteristic impedance of the air. The reverberation time can thus be rendered independent from low to medium frequencies as in the Only in the case ofhigh, frequencies does it drop to a small value which can sc'arcely be influencedaby the variation. By using;

The reverberation-timecan, in the case of low frequenciesfor which the greatest varia tion is required'-be varied up to .a ratio of 30:1 and.

thicker layers of absorbent material with lower flow resistance the attenuation can be increased, especially in the case of low frequencies; the same effect can be attained by supplementing the porous material with plate resonators or perforated resonators.

In the case of high frequencies the absorption coefficient can be reduced by arranging perforated or slotted plates with a large degree of perforation or an extremely thin foil before the porous material, in order to allow the reduction in the reverberation time to commence first at higher frequencies.

For varying the reverberation time the porous plate or other absorbing means is or are moved parallel to the reverberation plate or folded about an axis near one edge of the plate preferably by means of a mechanical device. The movement can be carried out by hand or by remote control, for example, from a control room. This also applies for the other means of variation mentioned above.

The reverberation arrangement according to the invention is not very sensitive to noise interference as no actual sound wave radiation takes place. Nevertheless it must be provided with a sound-proof casing if the surrounding room is not very quiet. It can be protected against interference by structure-borne sound from outside and against the mechanism for regulating the reverberation by single or multiple resilient bearings.

The electric power required for exciting the plate only amounts to a fraction of that required in an ordinary sound reverberation chamber. This is also one of the advantages of the arrangement according to the invention in addition to the excellent frequency curve which can be obtained in the case of dynamic excitation and piezoelectric pick-up without or with only slight attenuation on the transmitter or reception side. It is advisable to arrange an adjustable high and low frequency accentuation and deaccentuation on one or both sides so as to obtain timbre effects and to a certain extent to enable an apparent change of the frequencyresponse curve of the reverberation time and of the liveness.

Some of the details of the invention will be elucidated with the aid of the accompanying drawings.

In Fig. 1 the metal reverberation plate 1 is fixed on a frame 2 by means of steel wires 3 which can be stretched with the aid of supplementary stretching devices, such as screws.

Fig. la is a sectional view la-la.

In Fig. 2 the plate 1 has a curved edge. It is spotwelded on the frame 2 at the points 4 so that it only touches the frame at these points.

2 Fig. 2a is a sectional view of Fig. 2 along the line Fig. 3 is a section of the arrangement according to the invention. The plate 1 is connected to the frame 2 at the welds 5. It is excited to vibrations by means of moving coil 6 rigidly connected to the plate 1, for example, by cementing, and moving freely in the air gap of the permanent magnet 7. A flexural crystal pick-up 8, rigidly mounted on the plate 1 with an interposed spaceir 9, serves as microphone with structure-borne soun Figs. 4 to 6 illustrate diagrammatically the main possibilities of additional attenuation and consequently the variations of the reverberation time. In Fig. 4 a rubber sheet 12 is hung on a frame 13. In raised position it does not touch the plate. When the frame is lowered, which is preferably arranged so that it is not parallel to the edge of the plate, the sheet first comes into contact at one end of the upper edge of the plate, and

of Fig. 1 along the line then the length of the line of contact gradually increases up to the entire length of the plate or sheet, whereby the smallest reverberation time is attained. The sheet is then stretched at the end first coming into contact.

Fig. 5 shows a liquid attenuation of the plate. The teeth 16 of the plate 1 first dip into the liquid 15 when the vessel 14 is raised. As the plate dips further into the liquid the length of the immersed serrated edge increases and the reverberation time decreases until the teeth are completely submerged.

Fig. 6 shows diagrammatically the attenuation of the plate 1 suspended on wires 3 on the frame 2, by means of a porous sound absorbing sheet 17 which is mounted on a frame 18. On approaching the sheet, the reverberation time of the plate becomes less. The attenuation effect is increased twofold by providing a second porous sheet on the other side of the plate and at the same dis tance therefrom as the first sheet.

I claim:

1. An acoustic arrangement for the production of artificial reverberation within a predetermined range of sound frequencies comprising a substantially undamped plate suspended only at several points, the surfaces of the plate being smooth and its length and width being so large compared to its thickness that they are equal to at least two bending wavelengths at the lowest frequency Within the predetermined range of frequencies, means for exciting said plate over the predetermined range of frequencies to vibrate in a bending mode and means cooperating with said plate for adjustably damping the vibrations thereof in said mode.

2. An acoustic reverberation arrangement as in claim 1, in which the means for supporting the plate includes a rigid frame, and non-reverberant wires extending under tension between the edges of the plate and the frame.

3. An acoustic reverberation device as in claim 2 in which the means for damping the vibrations of the plate comprises a rubber sheet movably contacting an edge of the plate.

4. An acoustic reverberation device as in claim 2 in which the edge of the plate is serrated and the means for damping the vibrations of the plate comprises a container having a liquid therein in which at least a portion of the serrations are immersed.

5. An acoustic reverberation device as in claim 2 in which the means for damping the vibrations of the plate comprises at least one layer of sound absorbing porous material movably mounted for adjustment toward and from a surface of the vibratile plate and lying in a plane parallel thereto.

6. An acoustic reverberation device as in claim 2 in which said means for exciting vibrations of the plate in the bending mode comprises a driving member in direct engagement with the plate.

7. An acoustic reverberation device as in claim 2 in which said means for exciting vibrations of the plate in the bending mode comprises a driving member in direct engagement with the plate and means adjacent the plate for picking up and translating the vibrations produced by the plate.

References Cited in the file of this patent UNITED STATES PATENTS 1,843,553 Gladstone Feb. 2, 1932 2,069,254 Kunze Feb. 2, 1937 2,230,836 Hammond Feb. 4, 1941 2,437,445 Stack Mar. 9, 1948 2,512,130 Arenberg June 20, 1950 2,703,504 Rowe Mar. 8, 1955 2,727,423 Meeker Dec. 20, 1955 

